Heart rate, blood pressure, and oxygen consumption were measured in 27 hyperactive children during rest, exercise, and recovery, once taking placebo and again taking methylphenidate hydrochloride. Half were measured first taking the drug and half first taking the placebo. Twenty-three matched controls were also measured twice to test for reproducibility of results. Drug and placebo ECGs were recorded on 12 of the subjects. Oxygen consumption did not change (P = .40), but heart rate (P = .001) and blood pressure (P = .003) increased significantly with methylphenidate therapy. There was a significant correlation between size of dose in milligrams per kilogram and increase in heart rate (.38, P less than .05) and blood pressure (.50, P less than .05 systolic) (.46, P less than .05 diastolic). No evidence of the development of tolerance to these drug effects was found in children who had been taking methylphenidate from two months to more than a year. No ECG changes other than tachycardia were seen.